The present invention relates generally to an electrolytic cell of the filter press type wherein a series of bipolar electrodes with diaphragms or membranes sandwiched in between can be used for electrochemical production of alkali metal hydroxides and halogens. More particularly, the present disclosure relates to an improved method for connecting the backplates of the bipolar electrodes by welding a metal laminate strip therebetween to provide the essential electrical and mechanical connection while leaving sufficient air space to allow hydrogen gas to escape from within the cell, preventing hydrogen embrittlement of the titanium anode backplate.
Chlorine and caustic (sodium hydroxide) are essential and large volume commodities which are basic chemicals required in all industrial societies. They are produced almost entirely by the electrolysis of aqueous solutions of alkali metal chlorides, with a major proportion of current production coming from the diaphragm type electrolytic cells. These cells generally have a plurality of electrodes disposed within the cell structure to present a plurality of rows of alternatively spaced anodes and cathodes. These electrodes are generally foraminous in nature and made of a mesh or expenaded metal material so that a hydraulically permeable diaphragm may be formed over the cathode. This compartmental cell structure allows fluid flow through the cell. Brine (sodium chloride solution) starting material is continuously fed into the cell through the anode compartment and flows through the diaphragm backed by the cathode. To minimize back-diffusion and migration through the hydraulically permeable diaphragm, the flow rate is always maintained in excess of the conversion rate so that resulting catholyte solution has unreacted alkali metal chloride present. This catholyte solution, containing sodium hydroxide, unreacted sodium chloride, and certain other impurities, must then be concentrated and purified to obtain a marketable sodium hydroxide commodity and a sodium chloride solution to be reused in the diaphragm electrolytic cell. This is a serious drawback since the costs of this concentration and purification process are rising rapidly.
With the advent of technological advances such as the dimensionally stable anode which permits ever narrowing gaps between the electrodes and the hydraulically impermeable membrane, other electrolytic cell structures are being considered. The geometry of the diaphragm cell structure makes it inconvenient to place a planar membrane between the electrodes, hence the filter press electrolytic cell structure with planar electrodes has been proposed as an alternate electrolytic cell structure.
A filter press electrolytic cell is a cell consisting of several units in series, as in a filter press, in which each electrode, except the two end electrodes, acts as an anode on one side and a cathode on the other, and the space between these bipolar electrodes is divided into anode and cathode compartments by a membrane. In a typical operation, alkali metal halide is fed into the anode compartment where halogen gas is generated at the anode. Alkali metal ions are selectively transported through the membrane into the cathode compartment, and combine with hydroxyl ions generated at the cathode by the electrolysis of water to form the alkali metal hydroxides. In this cell the resultant alkali metal hydroxide is sufficiently pure and concentrated to be commercially marketable, thus eliminating an expensive second step of processing. Cells where the bipolar electrodes and the diaphragms or membranes are sandwiched into a filter press type construction may be electrically connected in series, with the anode of one connected with the cathode of an adjoining cell through a common structural member of partition. This arrangement is generally known as a bipolar configuration. A bipolar electrode is an electrode without direct metallic connection with the current supply, one face of which acts as an anode and the opposite face as a cathode when an electric current is passed through the cell.
While the bipolar configuration provides a certain economy for electrical connection of these electrodes in series there is a serious problem with the corrosion of cell components in contact with the anolyte. The anolyte normally contains highly corrosive concentrations of free halide, and the use of base metals such as iron to contain the solution have proven to be ineffective.
Proposals to overcome this problem include utlizing valve metals or alloys thereof to contain anolyte, either by fabricating an entire electrode from such a corrosion resistant material or by bonding a coating of valve metal onto a base metal within the anolyte compartment. The use of large quantities of expensive valve metals in commercial cell construction though has proven to be economically undesirable. The coated base metals on the other hand are prone to distintegration by peeling off of the protective layer and have also proven ineffective. It has been found that use of an air space between the backplates will act as an insulation against hydrogen ion travel and the resulting hydrogen embrittlement, because the hydrogen ions combine to form molecular hydrogen more readily than the ions move through the air space. Molecular hydrogen can then be simply vented off. This provides a convenient means for solving the embrittlement problem but leaves the problem of properly connecting the backplates in parallel spaced relation to each other. Welding would be ideal except that heretofore only insufficient methods were available for welding different metallic materials together such as steel and titanium.
Electrical and mechanical connection of these bipolar electrodes has been accomplished by internal bolting systems wherein the electrode is bolted through one pan, providing a spaced relation by use of a spacer of some sort, and through the second pan to the other electrode. Another method employs the use of an external bus-bar, outside of the electrolytic cell structure. Electrical connections made by the internal bolting system are undesirable because elaborate sealing schemes are necessary to prevent electrolyte leakage which could result in an extreme corrosion of the cathode compartment. This increases the cell costs and necessitates frequent maintenance. Electrical connections made externally are also not desirable since larger power losses are occasioned by the added structural voltage drops.
Thus it has become exceedingly advantageous to provide a method for connecting the bipolar electrode backplates in a spaced relation at a commercially viable cost.